inSync reader Dennis C. asks: "Digital "language" aside (S/PDIF, ADAT, etc.), there are a number of different physical connections for digital routing (RCA, coax, optical, etc.). Are there any performance differences or other issues to be aware of?"
Of course. RCA and coax are for all practical purposes the same thing as it relates to digital i/o. They are both supposed to be a 75 ohm coax type cable, not an audio RCA cable. However, an audio cable often works in a pinch. The XLR type connection, typically associated with AES, is a balanced signal and is capable of traveling farther than the coax style with more stability. The spec I remember for AES on an XLR balanced line was 300 feet. This assumes that you are using very high quality cable for the job, not a standard microphone cable. An unbalanced coax line shouldn't be used to go more than about 15 or 20 feet max. Of course, the usable length is always a function of the acceptable error rate, the compliance of the manufacturers to the specification, the type of cable used, and the quality of connectors on the cable as well as the device at each end. The shorter length the better.
Optical comes in two variations, though we typically see only one of them: Plastic optical cable is rated for about 30 feet according the folks at Alesis. This is a snag that Yamaha 02R users have encountered when trying to interface a board digitally to ADAT's. One can have real glass optical cable made which can travel much further, but it's really expensive. Optical does have two advantages over the other formats: 1) It is not susceptible to EMI (see WFTD archive EMI), and 2) There is no way for it to contribute to a ground loop problem since it does not create an "electrical" connection between devices.
In conclusion I would say the XLR balanced connections and the optical connections are the most robust. But they all are going to be affected by jitter (see WFTD archive Jitter), transmission length difficulties, and in the case of XLR and coax they can contribute to, and be affected by, ground loops and other types of electrical interference.
